Silicon pressure sensors offer multiple benefits that include small size, good quality, and stable performance. Further, since multiple identical sensors can be fabricated simultaneously on a single wafer, silicon pressure sensors are also cost effective to manufacture. In at least one example of a pressure sensor, piezo-resistors are fabricated on a silicon diaphragm such that the piezo-resistors sense the strain in the diaphragm as the diaphragm reacts to pressure applied by the pressure media. Even when the pressure media is clean dry air, the silicon sensing die requires some form of isolation from the environment. For absolute sensors this is provided by the normal SiO2 layer that is formed. The clean dry air is contained to only contact the side of the diaphragm that does not contain metal traces and pads. That surface is exposed only to a reference vacuum. For precision applications where the pressure media is limited to air, the silicon diaphragm can be mounted on a stress isolation member.
In one embodiment, the stress isolation member is a Pyrex tube. The Pyrex tube surface is hygroscopic in nature and absorbs H2O from the pressure media over time. This change in water content alters the tube geometry slightly, which cause the sensing element to drift. In addition to long term drift due to differences in the coefficient of expansion of silicon and Pyrex, and due to stresses produced during assembly processes, the pressure sensors need extensive conditioning to remove early drift components. Finally, unwanted leakage currents between the piezo-resistors and the housing of the sensor or external electrical connection can occur. The thin coating of silicon oxide that naturally forms is often relied on to provide electrical isolation. This film also can absorb water, which also may produce sensor drift.